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Coastal communities in British Columbia and Alaska were evacuated to higher ground early this morning after tsunami warning sirens blared following a large earthquake off the coast of Alaska. But the warning was later cancelled without any reported tsunami damage. Why?
Tsunamis are giant waves — up to 30 metres high — generated by large earthquakes, and can cause massive damage. The 2004 Boxing Day tsunami in the Indian Ocean killed more than 200,000 people. More recently, a massive earthquake and tsunami in Japan in 2011, caused major damage and a meltdown at the Fukushima nuclear power plant.
Tuesday’s earthquake struck at 1:31 a.m. PT in the Pacific Ocean about 250 kilometres southeast of Chiniak, Alaska. The U.S. Geological Survey initially reported the quake’s strength at 8.2 and later revised that to 7.9, with a depth of 25 kilometres. At least three aftershocks were reported.
According to the International Tsunami Information Centre, the most destructive tsunamis are generated from large, shallow earthquakes — those with a magnitude higher than 7.5 and less than 70 kilometres deep — with an epicentre or fault line near or on the ocean floor.
Tuesday’s earthquake met those criteria.
“This kind of earthquake was what is called a strike-slip earthquake,” Wagstaffe said. “That means there isn’t as much vertical displacement. So when you’re thinking of these rocks on the ocean floor, it didn’t punch up like some other earthquakes have.”
In the case of the 2004 and 2011 earthquakes, there was more vertical motion, which had devastating consequences. Roughly 225,000 were killed in the 2004 earthquake; 20,000 people were killed in Japan in 2011.
The West Coast is an area with quite a few fault lines that can result in various motions, Wagstaffe explains.
“Where the Gulf of Alaska curves, you end up getting a splay of fault lines,” she said. “There’s a really interesting section where this earthquake occurred where you can get all different kinds of motion happening. If it had’ve occurred 90 kilometres to the west then it would have been a subduction earthquake and we could’ve seen a massive tsunami.”
And it’s not without precedent.
On March 27, 1964, a 9.2 magnitude earthquake occurred off the coast of Alaska at a depth of 25 kilometres. Rather than being a strike-slip, it was a megathrust and produced a tsunami that rose 30 metres in some areas, killing more than 130 people.
“This is a reminder that we live in a seismically active area of the world.… It’s also a reminder that we get different kinds. We’re waiting for the ‘Big One,’ but we could also get a smaller earthquake, shallower closer to a major city. That would be devastating.”
“We are quite vulnerable to a lot of different earthquake risks here.”